While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322–334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm ) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610–3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material—dopant, which results in an efficiency improvement and allows color tuning.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes.
A useful class of dopants is that derived from 5,6,11,12-tetraphenylnaphthacene, also referred to as rubrene. The solution spectra of these materials are typically characterized by wavelength of maximum emission, also referred to as emission λmax, in a range of 550–560 nm and are useful in organic EL devices in combination with dopants in other layers to produce white light. Use of these rubrene-derived dopants in EL devices depends on whether the material sublimes. If the material melts, its use as a dopant is limited. Sublimation and deposition are the processes by which the dopant, subjected to high temperature and low pressure passes from the solid phase to the gas phase and back to the solid phase and in the process is deposited onto the device. Depending on the chemical structure of the dopant, when the temperature needed to sublime the dopant is high, thermal decomposition can occur. If the decomposition products also sublime the device can become contaminated. Decomposition leads to the inefficient use of dopant. Contamination with decomposition products can cause the device to have shorter operational lifetimes and can contribute to color degradation and light purity. In order to achieve OLEDs that can produce high purity white light, have good stability and no contamination from dopant decomposition, in addition to efficient use of dopant, one needs to have the ability to lower the sublimation temperature.
Useful dopants are those that emit light in ethyl acetate solution in the range of 530–650 nm, have good efficiency and sublime readily.
U.S. Pat. No. 6,387,547; U.S. Pat. No. 6,399,223; EP 1,148,109A2, and JP20001156290A teaches the use of rubrene derivatives containing either 2 phenyl groups on one end ring of the rubrene structure or 4 phenyl groups on both end rings. There is no teaching of fluorine or fluorine-containing groups on the rubrene structure.
JP 1998289786A discloses compound “15” with two fluorine-containing groups on the 5- and 12-positions of the naphthacene nucleus. Compound 15 falls outside the scope of the current invention.
WO 02/100977A1 discloses compound “C12” with two fluorine-containing groups also on the 5- and 12-positions of the naphthacene nucleus, but this too falls outside the scope of the current invention.
JP 04335087 discloses specific compounds 6, 13 and 14 containing chlorine or bromine at various positions on the rubrene molecule.
However, high sublimation temperatures and possible decomposition would limit the use of these rubrene derivatives. Thus devices containing these rubrene derivatives would fail to provide consistent white OLED devices with high color purity and reduced potential for possible contamination from decomposition impurities in their deposition.
The problem to be solved is to provide a dopant compound for a light-emitting layer of an OLED device that provides good luminance efficiency and low sublimation temperatures.